The present invention relates to a device for purifying an exhaust gas, a method for purifying an exhaust gas, a catalyst for purifying an exhaust gas, and a method for manufacturing the catalyst, arranged in such a manner that a NOx (nitrogen oxide) absorber that absorbs NOx in an excess oxygen atmosphere is disposed in an exhaust gas passage of an engine or the like, thereby making it possible to remove NOx from exhaust even in a lean air-fuel ratio condition.
Generally, there has been a known arrangement to dispose, in an exhaust gas passage of an engine, a NOx absorber that absorbs NOx in an exhaust gas when an air-fuel ratio mixture is lean, and hence, when an oxygen concentration in the exhaust gas is high, and releases NOx when the oxygen concentration drops, thereby purifying released NOx by means of reduction.
However, in a case where fuel or engine oil contains a slight amount of sulfur component (S), when the sulfur component are burned and exhausted, a conventional NOx absorber absorbs SOx (sulfur oxide) more readily than NOx in an exhaust gas, and further, once it has absorbed SOx, it hardly releases SOx even when the oxygen concentration in the exhaust gas drops. As a result, an amount of absorbed SOx increases with a lapse of time while the NOx absorption ability gradually deteriorates.
In regard to a problem of the S poisoning (sulfur poisoning), Japanese Patent Laid-Open Publication No. 6-142458 describes that combining Ba serving as a NOx absorber with at least one of alkali metal, Fe, Ni, Co, and Mg and supporting the combination on a substrate is advantageous in improving resistance to the S poisoning of Ba, and that in the case of a combination of Ba and K, SO2 in the exhaust gas is incorporated in the form of complex sulfate with Ba and K, and when the oxygen concentration drops, the complex sulfate decomposes or is reduced at low temperatures to BaO and K2O that are active to NOx, which provides an advantage in improving resistance to the S poisoning of Ba. It should be noted, however, that the Ba support amount per 1 L (an apparent volume, and the same applies to the description below) is 13.7 to 27.4 g and the K support amount per 1 L is 0.39 to 7.8 g of the substrate.
However, when the K support amount is too little, the NOx absorption ability is not improved, and when the K support amount is increased to improve the NOx absorption ability, HC is not purified adequately in an atmosphere with reductants (excess oxygen ratio xcexxe2x89xa61), that is, during a theoretical air-fuel ratio combustion operation or a rich combustion operation of the engine. The reason for this is not obvious, but it is assumed that a great deal of K is placed in the periphery of precious metal and prevents HC from approaching in close proximity to the precious metal.
Japanese Patent Laid-Open Publication No. 7-51544 describes that when a combination of at least two kinds of alkaline-earth metal is supported on a substrate as a NOx absorber, the NOx absorber incorporates SO2 in the exhaust gas as complex sulfate and the complex sulfate readily decomposes at low temperatures when the oxygen concentration drops, thereby providing an advantage in improving resistance to the S poisoning of Ba. It should be noted, however, that, in the case of a combination of Ba and Mg, the Ba support amount per 1 L of the substrate is 41 to 69 g and the Mg support amount per 1 L is 2.4 to 4.8 g, and in the case of a combination of Ba and Sr, the Ba support amount per 1 L of the substrate is 41 to 69 g and the Sr support amount per 1 L is 8.7 to 42 g.
Also, Japanese Patent Laid-Open Publication No. 10-118494 describes that, in regard to a NOx purifying catalyst for purifying NOx by means of reduction in an excess oxygen atmosphere, by supporting Pt and Rh as catalytic metal on an alumina substrate and by also supporting Sr or Mg in addition to K having high affinity to NOx, it is possible to achieve high NOx purifying performance even in the presence of SOx. It is preferable to arrange in such a manner that, in the case of a combination of K and Sr, the K support amount per 1 L of the substrate is 20 to 40 g and the Sr support amount per 1 L is 0 to 50 g, and in the case of a combination of K and Mg, the K support amount per 1 L of the substrate is 5 to 20 g and the Mg support amount per 1 L is 0 to 5 g.
Also, Japanese Patent Laid-Open Publication No. 10-274031 describes that SOx absorbed in the NOx absorber is detached by injecting fuel in an expansion stroke of an in-cylinder direct injection engine and thereby raising a temperature of the exhaust gas.
The object of the present invention is to control absorption of sulfur component in an exhaust gas by the NOx absorber.
Another object of the present invention is to improve the heat resistance of the NOx absorber.
A further object of the present invention is to achieve a balance between NOx purifying performance during a lean combustion operation after being exposed to a high temperature atmosphere and HC purifying performance during a theoretical air-fuel ratio combustion operation or a rich combustion operation by adequately setting the K support amount.
Still another object of the present invention is to regenerate deteriorated NOx absorption ability of the NOx absorber resulted from absorption of sulfur component in the exhaust gas by causing the NOx absorber to detach the sulfur component, and more particularly to make an arrangement of the NOx absorber such that it can readily regenerate by detaching the sulfur component when predetermined sulfur detaching means is activated.
The present invention provides a catalyst for purifying an exhaust gas, disposed in an exhaust gas passage of an engine, for lowering NOx concentration in an exhaust gas containing NOx, sulfur, and oxygen, the catalyst including:
a substrate; and
a catalyst layer formed on the substrate by supporting, on alumina, a NOx absorber that absorbs NOx when an oxygen concentration in the exhaust gas is high and releases NOx when the oxygen concentration drops, and a precious metal for reducing NOx,
which is characterized in that the NOx absorber includes Ba, K, Sr, and Mg.
According to the invention, deterioration of the NOx absorption ability of the NOx absorber caused by the S poisoning can be suppressed, and the heat resistance of NOx absorber can also be improved. The reason for this is not obvious, but it is assumed as follows.
Firstly, it is assumed that the elements (K, Sr, and Mg) other than Ba are more susceptible to the S poisoning than Ba, and for this reason, the S poisoning of Ba is relatively small. In other words, because Ba has the higher NOx absorption ability than the other elements, the presence of the other elements makes the S poisoning of Ba relatively small, the deterioration of the NOx absorption ability is lessened.
According to an analysis, it appears that Ba and Sr (at least a part of each) form one compound (a mixed oxide or double salt) with both the elements being constituent elements. It is assumed that such a Baxe2x80x94Sr compound (hereinafter, referred to as a double compound as needed) is less susceptible to the S poisoning compared with the case of Ba alone, and for this reason, deterioration of the NOx absorption ability can be suppressed.
Also, according to an analysis, it appears that Ba and Mg (at least a part of each) do not form a crystal, but are in an almost amorphous state by coming in close proximity to each other or bonding to each other. It is assumed that such a Baxe2x80x94Mg coexisting substance suppresses the S poisoning of Ba (formation of barium sulfate) compared with the case of Ba alone, and for this reason, deterioration of the NOx absorption ability can be suppressed.
Also, it is affirmed from analysis that K neither forms a complex nor undergoes affinity with Ba, Sr, and Mg, and resides dispersedly in the periphery of the Baxe2x80x94Sr compound or Baxe2x80x94Mg coexisting substance. K with the foregoing nature is relatively highly reactive with sulfur, and therefore, is assumed to prevent the S poisoning of Baxe2x80x94Sr compound or the Baxe2x80x94Mg coexisting substance. In addition, K promotes crystallization of Baxe2x80x94Sr double carbonate and activates the NOx absorber, thereby contributing to the improvement of the heat resistance of the catalyst.
It is also assumed that interaction of the elements forming a quaternary material of Baxe2x80x94Kxe2x80x94Srxe2x80x94Mg serving as the NOx absorber weakens the bond to SOx, so that even if SOx is bonded, it is readily detached.
Further, when the NOx absorber 25 is made of a single element of Ba, if an amount of Ba is increased, a particle size becomes larger but a specific surface area hardly increases. However, in a case where Ba and other elements (K, Sr, and Mg) are combined, when the amount of the elements are increased, it is assumed that a particle size hardly increases, but a specific surface area, or an active site, increases, which makes an absorption capacity of NOx and SOx larger. Hence, even when the S poisoning occurs to some degree, the NOx absorption ability is not deteriorate significantly.
Also, combining Ba and the other elements (K, Sr, and Mg) as discussed above is effective in making the NOx absorber into fine particles. In particular, Sr is notably excellent in making Ba and Mg into fine particles. Hence, the NOx absorber is dispersed on the substrate at a high degree, which makes the occurrence of heat sintering difficult. In short, the heat resistance of the catalyst becomes high.
Also, the reason why alumina is used as a support material of the above-discussed NOx absorber and precious metal is because alumina hardly undergoes sintering nor decomposes when it is heated high, and therefore, is advantageous in preventing heat deterioration of the catalyst. In the case of alumina, Ba and the substrate react with each other when the catalyst is heated high, which readily causes deterioration, but Mg suppresses the reaction between the substrate and Ba, and as a result, heat deterioration of the catalyst is prevented.
The support material can be made of both alumina and a ceria material. The ceria material serves as an oxygen storage material, and releases oxygen when oxygen concentration in the exhaust gas drops, thereby promoting a ternary reaction (oxidation-reduction reaction) among HC (hydrocarbon), CO, and NOx in the exhaust gas. In addition, it is advantageous to increase an amount of the ceria material in improving resistance to the S poisoning of the catalyst. In particular, a ceria material containing Zr increases the resistance to the S poisoning.
In the case of a ternary catalyst, an addition alumina added with Ba, Zr, La, etc. is used occasionally as alumina to suppress reduction of a specific surface area when exposed to a high temperature. However, using non-addition alumina that does not contain these additional elements is advantageous to NOx purification at a lean ratio. In other words, the precious metal at a lean ratio serves as a catalyst to oxidize NO in the exhaust gas to NO2, and assists the NOx absorber to absorb NOx. The alumina serves to assist this catalytic reaction of precious metal, but when an additive is present as discussed above, a heat resistance is improved but the function of alumina as a co-catalyst is deteriorated. For this reason, a non-addition alumina is advantageous to NOx purification at a lean ratio.
The ceria material may be composed of CeO2 alone, but it may be a mixed oxide of Ce and Zr discussed above. Further, it may be a ternary mixed oxide of Cexe2x80x94Zrxe2x80x94Sr. Adopting the ternary complex oxide of Cexe2x80x94Zrxe2x80x94Sr is advantageous in improving the heat resistance, resistance to the S poisoning, regeneration properties from the S poisoning of the catalyst, which will be discussed below.
It is preferable to combine alumina with the ceria material at a mass ratio of 1:1 or in the vicinity thereof, which provides an advantage in improving both heat resistance and resistance to the S poisoning of the catalyst.
It is preferable to use Pt as the precious metal, because it shows an excellent catalytic function for the oxidation of NO to NO2 at a lean ratio, and the reduction of NO2 to N2 at stoichimetric or rich ratio. Also, it is more preferable to use both Pt and Rh. Rh serves to assist the catalytic reaction by Pt, in other words, it promotes the above-discussed ternary reaction at a stoichimetric or rich ratio while promoting a reduction-decomposition reaction of NOx released from the NOx absorber. When the support amount of Rh per 1 L of the substrate is in a range from 0.1 to 1.0 g, variance of the amount does not make a significant difference in the NOx purification ratio, and therefore, a small amount is sufficient.
A preferable support amount of Pt per 1 L of the substrate is 1 to 15 g. This is because when the amount is less than 1 g, NOx is not sufficiently purified by means of reduction, whereas when the amount exceeds 15 g, no further improvement of the NOx purification ratio is expected and the costs is increased. It is sufficient to set the Rh support amount to approximately {fraction (1/10)} to {fraction (1/100)} of the Pt support amount.
In the above catalyst for purifying an exhaust gas, the support amount of Sr per 1 L of the substrate is preferably 8 to 20 g, and the support amount of Mg per 1 L of the substrate is preferably 5 to 15 g, and more preferably 8 to 12 g.
The above arrangement makes it possible to achieve an effect in improving resistance to the S poisoning by Mg and Sr while achieving an effect in improving the heat resistance by Mg as discussed above. The support amount of Ba per 1 L of the substrate is preferably 25 to 60 g.
In the above catalyst for purifying an exhaust gas, the mass ratio of Ba, Sr, and Mg in the catalyst layer is preferably Ba:Sr:Mg=30:(8 to 20):(8 to 12).
This provides an advantage in improving the heat resistance of the NOx absorber while suppressing the S poisoning of the NOx absorber.
In the above catalyst for purifying an exhaust gas, the mass ratio of Ba, K, Sr, and Mg in the catalyst layer is preferably Ba:K:Sr:Mg=30:(2 to 12):(8 to 20):(8 to 12).
This provides a further advantage in improving the heat resistance of the NOx absorber while suppressing the S poisoning of the NOx absorber.
In the above catalyst for purifying an exhaust gas, the support amount of K per 1 L of the substrate is preferably 2 to 12 g.
In other words, the above-discussed promotion of crystallization of Baxe2x80x94Sr double carbonate by K, and the effect of improving the heat resistance of the catalyst resulted therefrom are exerted when the support amount of K is 2 g/L or more. It should be noted, however, that when the support amount of K exceeds 12 g/L, the exertion of the effect becomes weak. In this case, a more preferable support amount of K is 4 to 10 g/L.
Also, in the above catalyst for purifying an exhaust gas, the support amount of K per 1 L of the substrate is preferably 2 to 6 g.
In other words, because the support amount of K per 1 L of the substrate is set to 6 g/L or less, when the oxygen concentration in the exhaust gas drops after being exposed to a high temperature atmosphere (when an atmosphere with reductants (xcexxe2x89xa61) is reached), it is possible to suppress deterioration of HC oxidation-purifying performance due to the precious metal.
Also, because the support amount of K per 1 L of the substrate is set to 2 g/L or more, it is possible to achieve the above-discussed effects in preventing the S poisoning of Ba, Mg and Sr by K. Hence, it is possible to purify NOx released from the NOx absorber when the lean combustion operation is switched to the theoretical air-fuel ratio combustion operation or rich combustion operation by reacting NOx with HC in a satisfactory manner.
In case that the support amount of K per 1 L of the substrate is 2 to 6 g, a mass ratio of Ba and K in the catalyst layer is preferably Ba:K=(5 to 15):1.
In other words, because the mass ratio of the support amount of Ba to the support amount of K is set to 5 or higher, the NOx absorption ability will not be deteriorated because the support amount of Ba is too small. Also, because the mass ratio is set to 15 or less, neither the NOx absorbing site of Ba will be decreased by sintering caused when the catalyst is calcined, nor Ba will be crystallized on the substrate and separated therefrom because the support amount of Ba is too large.
Hence, it is possible to properly perform the function to allow NOx released from Ba to react with HC in a satisfactory manner when the oxygen concentration becomes high (at the theoretical air-fuel ratio combustion operation or rich combustion operation) without deteriorating the NOx absorption ability of Ba when the oxygen concentration in the exhaust gas is high (at lean combustion operation of the engine).
In the above catalyst for purifying an exhaust gas, when the oxygen concentration in the exhaust gas is high means when the oxygen concentration is, for example, 5% or higher.
In the above catalyst for purifying an exhaust gas, the engine may be a lean burn gasoline engine or a diesel engine.
Also, the present invention provides a method of manufacturing a catalyst for purifying an exhaust gas, disposed in an exhaust passage of an engine, for lowering the NOx concentration in an exhaust gas containing NOx, sulfur, and oxygen, characterized by including the steps of:
forming an alumina layer by coating a substrate with alumina; and
impregnating the alumina layer with a Ba solution, a K solution, a Sr solution, a Mg solution, and a precious metal solution.
This makes it possible to obtain an exhaust gas purifying catalyst in which the catalyst layer is formed on the substrate by supporting Ba, K, Sr, and Mg as NOx absorber and the precious metal for reducing NOx on alumina. Consequently, it is possible to improve the heat resistance of the NOx absorber while suppressing the S poisoning of the NOx absorber.
In the above method of manufacturing a catalyst for purifying an exhaust gas, each of the Ba solution, the K solution, the Sr solution, and the Mg solution may be a solution of acetic acid.
In the above method of manufacturing a catalyst for purifying an exhaust gas, it is preferable to form the alumina layer into a lamination by coating the substrate with the alumina twice, after which the Ba solution, the K solution, the Sr solution, the Mg solution, and the solution of the precious metal are impregnated into the two alumina layers.
In other words, in a case where a thick catalyst layer is formed on the substrate, if the substrate is coated with alumina at one time, an amount of alumina is so large that the thickness of the alumina layer readily becomes irregular. Also, it takes a time to dry and calcine the alumina layer. On the contrary, coating alumina twice separately as discussed above is advantageous in attaining the alumina layer with an even thickness, and it also shortens a drying and calcining time. Also, by providing a double-layer alumina layer, when the NOx absorber is impregnated, the concentration of the NOx absorber in the outer alumina layer is higher than that in the inner alumina layer, and therefore, SOx is chiefly trapped by the NOx absorber on the outer alumina layer, and the NOx absorber less damaged by the S poisoning can be secured on the inner alumina layer, which provides an advantage in maintaining the NOx purifying performance.
In the above method of manufacturing a catalyst for purifying an exhaust gas, it is preferable that the Ba solution, the K solution, the Sr solution, the Mg solution, and the solution of the precious metal are mixed and impregnated into the alumina layer simultaneously.
In other words, if the solution of the precious metal and the solutions of the NOx absorber are separated from each other, and the solution of the precious metal is impregnated first, then the precious metal is covered with the NOx absorber impregnated later and is readily buried in the alumina layer. On the other hand, if the solution of the precious metal is impregnated later, the NOx absorber impregnated and supporting first, especially Ba, dissolves into the solution of the precious metal, which causes unsatisfactory dispersion.
On the contrary, if the solutions are impregnated simultaneously like in the present invention, it is possible to place the precious metal in close proximity to the NOx absorber without burying the precious metal, and unsatisfactory Ba dispersion does not occur, which is advantageous to reduction-purification of NOx. Also, by the simultaneous impregnation of the solutions of four kinds of the NOx absorber, it is possible to efficiently form the above-described Baxe2x80x94Sr compound or Baxe2x80x94Mg coexisting substance and to disperse K in the periphery of these compounds. This is advantageous not only in suppressing the S poisoning of the NOx absorber, but also in making the NOx absorber into fine particles, in particular, in making Ba and Mg into fine particles by Sr, thereby making it possible to increase the heat resistance of the catalyst.
In the above method of manufacturing a catalyst for purifying an exhaust gas, in a case where the Ba solution, the K solution, the Sr solution, and the Mg solution are divided into two groups including a group which is impregnated into the alumina layer first, and a group which is impregnated later, it is preferable to impregnate the K solution later.
In other words, in a case where the Ba solution, the K solution, the Sr solution, and the Mg solution are impregnated into the alumina layer simultaneously, if too large amounts of Ba, K, Sr, and Mg are to be supported, the concentrations of these kinds of metals in their respective impregnation solutions becomes high, and for example, Ba with low solubility may not be dissolved and remain in the impregnation solution. In this case, impregnation of these metal components becomes uneven, which causes deterioration of the catalytic performance.
On the contrary, heating the impregnation solutions can raise the solubility of metal components, so that the entire metal components are dissolved without increasing a total amount of the impregnation solutions, but this requires a heating process. In order to solve this problem, the Ba solution, the K solution, the Sr solution, and the Mg solution are divided into two groups including a group which is impregnated into the alumina layer first, and a group which is impregnated later, and the K solution is impregnated later.
In this case, because K neither forms a complex nor undergoes affinity with the other NOx absorber, it is not essentially necessary to impregnate the K solution simultaneously with the other NOx absorber. On the contrary, impregnating the K solution later is advantageous in placing K in the periphery of the other NOx absorber and hence in improving the heat resistance of the catalyst.
In the above method of manufacturing a catalyst for purifying an exhaust gas, in a case where the Ba solution, the K solution, the Sr solution, and the Mg solution are divided into two groups including a group which is impregnated into the alumina layer first, and a group which is impregnated later, it is preferable to impregnate the Sr solution first.
In other words, because it is assumed that Sr makes Ba and Mg into fine particles as discussed above, making Ba and Mg into fine particles by having Sr be supported first is advantageous in increasing the heat resistance of the catalyst.
Also, the present invention provides a device for purifying an exhaust gas as shown in FIG. 1, comprising:
a NOx absorber 25, disposed in an exhaust passage 22 of an engine 1 or the like, for absorbing NOx and a sulfur component in an exhaust gas in an excess oxygen atmosphere where an oxygen concentration in the exhaust gas is high, and releasing NOx absorbed therein as the oxygen concentration drops;
sulfur excessive absorption determining means a for determining an excessive absorption state of the sulfur component in the NOx absorber 25; and
sulfur detaching means b for, when the sulfur excessive absorption determining means a determines the excessive absorption state of the sulfur component, causing the NOx absorber 25 to detach the sulfur component by raising a temperature of the NOx absorber 25 while lowering the oxygen concentration,
characterized in that at least one of K, Sr, Mg, and La, and Ba are included as elements forming the NOx absorber 25.
According to the above arrangement, by activating the sulfur detaching means b after the sulfur component (SOx) in the exhaust gas is absorbed excessively into the NOx absorber 25, the NOx absorption ability is readily regenerated to nearly the ability before the sulfur component were absorbed. In other words, the NOx absorber 25 has higher NOx absorption ability after the regeneration (regeneration from the S poisoning, and the same applies to the description below) or causes less deterioration of the NOx absorption ability when exposed to high temperatures, that is, the heat resistance becomes high, compared with a case where the NOx absorber is composed of Ba alone. This improvement of the heat resistance plays advantageously for the regeneration of the NOx absorber 25. The relation between the improvement of the heat resistance and the regeneration of the NOx absorber 25 is as follows.
In other words, the sulfur detaching means b causes the NOx absorber 25 to detach sulfur component not only by lowering the oxygen concentration in the exhaust gas, but also by raising the temperature of the NOx absorber 25. Hence, if the NOx absorber has a poor heat resistance, it becomes difficult to raise the temperature of the NOx absorber 25 for detaching the sulfur component, which makes it impossible to achieve the originally intended object. On the contrary, if the heat resistance of the NOx absorber 25 becomes high like in the present invention, it is possible to regenerate the NOx absorption ability by effectively using the sulfur detaching means b. In short, it is possible to avoid deterioration of the NOx absorber 25 by heat generated at the sulfur detaching treatment.
The reason why the present invention can make the NOx absorption ability after the regeneration higher or why it can improve the heat resistance compared with a case where the NOx absorber is composed of Ba alone is not obvious, but it is assumed as follows.
That is, it is assumed that any of the elements (K, Sr, Mg, or La) other than Ba is more susceptible to the S poisoning than Ba, and for this reason, the S poisoning of Ba is comparatively small, which lessens deterioration of the NOx absorption ability after the S poisoning. In other words, because Ba has higher NOx absorption ability than the other elements, the presence of the other elements makes the S poisoning of Ba relatively small, and thereby the deterioration of the NOx absorption ability is lessened.
It is also assumed that any of the other elements (K, Sr, Mg, or La) regenerates from the S poisoning more readily than Ba, and for this reason, the NOx absorption ability after the regeneration becomes high. In other words, although a sulfate of a produced compound of Ba and SOx is stable, but a sulfate of the other elements is less stable than sulfate of Ba. Hence, it is assumed that when a temperature rises high in a low oxygen concentration atmosphere, SOx is readily detached.
It is also assumed that formation of a complex by Ba with any of the other elements (Sr, Mg or La) except K (formation of a mixed oxide or double salt or turning into an almost amorphous state by coming in close proximity to each other or bonding with each other) makes the occurrence of the S poisoning difficult.
Further, when an amount of Ba is increased as a single element forming the NOx absorber 25, neither the NOx absorption ability before the S poisoning nor the NOx absorption ability after the regeneration is improved significantly. The reason for this is assumed that when an amount of Ba exceeds a certain amount, only a particle size is increased and a specific surface area remains the same. On the contrary, in a case where Ba and the other elements (at least one of K, Sr, Mg and La) are combined, it is assumed that each element resides separately because of a difference in their natures, which not only increases a specific surface area or a reaction site, but also makes the occurrence of heat sintering difficult. Further, it is assumed that the interaction of different elements forming the NOx absorber makes detachment of the sulfur component readily.
Also, combining Ba and the other elements (at least one of K, Sr, Mg and La) as discussed above is effective in making the NOx absorber into fine particles, and Sr is particularly effective in making Ba and Mg into fine particles. Consequently, the NOx absorber disperses on the substrate at a high degree, which makes the occurrence of heat sintering difficult. In short, the heat resistance of the catalyst becomes high.
Also, in case that the substrate is alumina, Ba and the substrate reacts with each other when the catalyst is heated high, which readily causes deterioration. However, because Mg suppresses the reaction between the substrate and Ba, the heat resistance of the catalyst is improved.
In a case where Ba and the foregoing other elements (at least one of K, Sr, Mg and La) are supported on a substrate of a honeycomb shape or the like, the support amount of Ba per 1 L of the substrate is approximately 10 to 50 g, and preferably 20 to 40 g, and the support amounts of the other elements are preferably as large as or less than the support amount of Ba.
The excess oxygen exhaust gas with high oxygen concentration is, for example, an exhaust gas (oxygen concentration is approximately 4 to 20%) when an engine is run on a lean mixture (particularly, A/F=18 to 50) with an air-fuel ratio A/F greater than 16.
As the elements forming the NOx absorber 25, it is preferable to include K in addition to Ba. According to this arrangement, the NOx absorption ability before the S poisoning becomes high. Also, because K does not form a complex with Ba, but is highly reactive with sulfur, K resides in the periphery of Ba and prevents the S poisoning of Ba, thereby suppressing deterioration of the NOx absorption ability caused by the S poisoning of Ba. Also, because it is assumed that K detaches the sulfur component more readily than Ba, the NOx absorption ability after the regeneration becomes high. A preferable mass ratio of Ba and K is, for example, Ba:K=30:(1 to 30).
As the elements forming the NOx absorber 25, it is preferable to include at least one of Sr, Mg, and La in addition to Ba and K. This makes the NOx absorber 25 have a high heat resistance, which is advantageous in avoiding heat deterioration at the sulfur detaching treatment.
Also, according to an analysis, it appears that Ba and Sr (at least a part of each) form one compound (mixed oxide or double salt) with both the elements being constituent elements. It is assumed that such a Baxe2x80x94Sr compound (hereinafter, referred to as a double compound as needed) is less susceptible to the S poisoning compared with the case of Ba alone, and for this reason, deterioration of the NOx absorption ability is suppressed.
Also, according to an analysis, it appears that Ba and Mg (at least a part of each) do not form a crystal, but are in an almost amorphous state by coming in close proximity to each other or bonding to each other. It is assumed that such a Baxe2x80x94Mg coexisting substance suppresses the S poisoning of Ba compared with the case of Ba alone, and for this reason, deterioration of the NOx absorption ability is suppressed.
Also, it is affirmed from analysis that K neither forms a complex nor undergoes affinity with Ba, Sr, and Mg, and resides dispersedly in the periphery of the Baxe2x80x94Sr compound or Baxe2x80x94Mg coexisting substance. K with the foregoing nature is relatively highly reactive with sulfur, and therefore, is assumed to prevent the S poisoning of Baxe2x80x94Sr compound or the Baxe2x80x94Mg coexisting substance.
In a case where Ba, K, and Mg are used as the elements forming the NOx absorber 25 and are supported on a substrate of a honeycomb shape or the like, it is preferable to set the support amount of Ba to 10 to 50 g, the support amount of K to 1 g or more (the upper limit is, for example, 15 g), and the support amount of Mg to 3 to 17 g per 1 L of the substrate. It is more preferable to set the support amount of Mg to 5 to 15 g and further 8 to 12 g. Consequently, the heat resistance can be obtained, and also regeneration properties from the S poisoning becomes satisfactory. A preferable mass ratio of Ba, K, and Mg is, for example, Ba:K:Mg=30:(1 to 30):(1 to 30).
In a case where Ba, K, and Sr are used as the elements forming the NOx absorber 25 and are supported on a substrate of a honeycomb shape or the like, the support amount of Ba and the support amount of K per 1 L of the substrate may preferably be equal to those in the above Baxe2x80x94Kxe2x80x94Mg-based absorber, and the support amount of Sr may be 10 to 20 g. The support amount of Sr may preferably be 13 to 17 g. Consequently, the heat resistance can be obtained, and also regeneration properties from the S poisoning becomes satisfactory. A preferable mass ratio of Ba, K, and Sr is, for example, Ba:K:Sr=30:(1 to 30):(1 to 30).
As the elements forming the NOx absorber 25, it is preferable to include Sr in addition to Ba. Thus, the NOx absorber 25 has a high heat resistance, which provides an advantage in avoiding heat deterioration during the sulfur detaching treatment.
As the elements forming the NOx absorber 25, it is preferable to include at least one of Mg and La in addition to Ba and Sr. Thus, the NOx absorber 25 has a high heat resistance, which provides a further advantage in avoiding heat deterioration during the sulfur detaching treatment.
As the elements forming the NOx absorber 25, it is preferable to include Mg in addition to Ba. Thus, the NOx absorber 25 has a high heat resistance, which provides an advantage in avoiding heat deterioration during the sulfur detaching treatment.
As the elements forming the NOx absorber 25, it is preferable to include La in addition to Ba and Mg. Thereby, the NOx absorber 25 has a high heat resistance, which provides a further advantage in avoiding heat deterioration during the sulfur detaching treatment.
Raising the temperature of the NOx absorber 25 by the sulfur detaching means b can be achieved by raising a temperature of an exhaust gas, and for example, increasing a temperature of the exhaust gas to 500 to 1100xc2x0 C. (preferably 600 to 1100xc2x0 C.) is advantageous to detachment of sulfur from the NOx absorber 25. Alternatively, a heater may be provided to the NOx absorber 25 so that the heater is heated. Also, lowering of oxygen concentration in the exhaust gas by the sulfur detaching means b can be achieved by controlling an air-fuel ratio in the engine, and for example, by setting xcex (oxygen-excessive ratio) to around 1 or to 1 or less, oxygen concentration in the exhaust gas becomes 0.5% or less, and further, an amount of reduction components, such as HC, CO, and H2, in the exhaust gas is increased, which is advantageous to detachment of the sulfur component from the NOx absorber 25.
In case that a spark ignition direct injection engine is used as the engine, the sulfur detaching means b is preferably fuel injection control means that operates a fuel injection valve to divide injection, so that fuel is injected into a combustion chamber in the cylinder at least twice from a start of an air-intake stroke and an end of a compression stroke. Consequently, it is possible to raise the temperature of the NOx absorber 25 by increasing the temperature of the exhaust gas while lowering the oxygen concentration in the exhaust gas. In particular, the divided injection as discussed above can raise CO concentration in the exhaust gas, which is further advantageous to detachment of the sulfur component from the NOx absorber 25.
More specifically, in a case where the NOx absorber 25 is Ba, it is assumed that SOx is adsorbed on the surface of barium particles in the form of sulfate, and barium sulfate undergoes the following reaction with a supply of CO, so that barium carbonate and sulfur dioxide are generated.
BaSO4+COxe2x86x92BaCO3+SO2↑ (coefficients omitted)
Meanwhile, a so-called water gas shift reaction between CO and water in the exhaust gas proceeds with increasing CO concentration, and as a result, hydrogen is generated at the reaction site on the catalyst.
CO+H2Oxe2x86x92H2+CO2
Then, hydrogen causes the sulfur component adsorbed in the NOx absorber 25 to be detached, which is advantageous to detachment of the sulfur component. Moreover, because the water gas shift reaction can proceed at a relatively low temperature, the temperature of the NOx absorber 25 does not have to be raised significantly.
Also, as the sulfur excessive absorption determining means a for determining a sulfur component excessive absorption condition of the NOx absorber 25, it is possible to, for example, adopt means arranged so that an amount of absorbed SOx in the NOx absorber 25 is estimated based on a travel distance of an automobile and a total amount of fuel consumed on that distant with a consideration given to the temperature conditions of the NOx absorber 25 during a corresponding period, and the sulfur component excessive absorption condition is determined when the estimated amount exceeds a predetermined value.
Also, the present invention provides an exhaust gas purifying method of purifying an exhaust gas containing NOx and sulfur component, characterized in that:
when the exhaust gas is in an excess oxygen absorption condition where an oxygen concentration is high, the exhaust gas is brought into contact with an NOx absorber 25 including at least one of K, Sr, Mg, and La, and Ba, so that NOx and the sulfur component are absorbed into the NOx absorber 25; and
when a sulfur component absorption condition of the NOx absorber 25 becomes a predetermined excessive absorption condition, the sulfur component are detached from the NOx absorber 25 by raising a temperature of the NOx absorber 25 and lowering the oxygen concentration in the exhaust gas.
In other words, according to the above method, as can be obvious from the foregoing description, when the NOx absorption ability of the NOx absorber 25 is deteriorated by the S poisoning, the NOx absorption ability can be readily regenerated to a high level by causing the NOx absorber 25 to detach the sulfur component, which is advantageous to NOx purification.
Also, the present invention provides an exhaust gas purifying catalyst for decreasing NOx in an exhaust gas from an engine that is run in such a manner that an exhaust gas therefrom contains sulfur and oxygen and oxygen concentration therein drops intermittently, comprising:
a substrate; and
a catalyst layer formed on the substrate by supporting, on alumina, a NOx absorber that absorbs NOx when an oxygen concentration in the exhaust gas is high and releases NOx when the oxygen concentration drops, and a precious metal for reducing NOx,
characterized in that the NOx absorber includes Ba, K, Sr, and Mg.
Hence, when an engine is run to increase the oxygen concentration in the exhaust gas, the NOx absorber absorbs NOx in the exhaust gas, and when the engine is run to lower the oxygen concentration in the exhaust gas, NOx is released from the NOx absorber, so that NOx is purified by means of reduction with the precious metal. The exhaust gas purifying catalyst as discussed above can obtain a heat resistance, and also is advantageous in improving the regeneration properties from the S poisoning.